The invention of the present application generally relates to methods and system for producing liquefied natural gas and, more particularly, to methods and systems for enhancing performance of natural gas liquefaction plants used to produce liquefied natural gas.
As will be appreciated, many large, naturally occurring reserves of natural gas are located in remote areas of the world. As one of the cleanest burning fossil fuels, this gas has considerable value if it can be economically transported to market. Where the gas reserves are found in reasonable proximity to a market, the gas is typically produced and then transported to market through submerged land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas (also “LNG”) to market in specially-designed storage tanks aboard transport vessels. The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure. To do this, the natural gas is cooled and condensed to a liquid state to produce LNG. Such LNG is typically transported at substantially atmospheric pressure and at temperatures of about −151° C. (−240° F.) to −162° C. (−260° F.), thereby significantly increasing the amount of gas which can be stored in a particular storage tank on a transport vessel. Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like. LNG has become an increasingly popular method of transporting natural gas to major energy-consuming customers.
Processing plants used to liquefy natural gas, which may be referred to herein as “liquefaction plants”, are typically built in stages as the supply of feed gas, i.e. natural gas, and the quantity of gas contracted for sale, increases. One traditional method of configuring a liquefaction plant is to build up the site in several sequential increments, or parallel “LNG production trains”. Each stage of construction may consist of a separate, stand-alone production train, which, in turn, is comprised of all the individual processing units or steps necessary to liquefy a stream of feed gas into LNG and send it on to storage. Each production train may function as an independent production facility. Production train size can depend heavily upon the extent of the resource, technology and equipment used within the train, the available funds for investment in the project development, and market conditions.
Operability and profitability of LNG plants during its life depends on gaining effective operational intelligence and then converting that into business intelligence. Establishing the process per design recommendations and/or controlling the process beyond design exposure due to varying operational dynamics presents a significant challenge for process engineers and, as will be appreciated, maintaining the plant at near optimal conditions is difficult. Thermodynamics and hydraulics constitute an important role in determining the gas-liquid ratio for optimized LNG production. Any imbalances can lead to significant energy and material losses. The associated cost of high specific energy consumed by refrigeration systems and/or the generation of excess flash gas in cryogenic processes can be considerable. In trying to optimize or enhance production efficiency and limit these costs, conventional systems have failed to connect upstream process dynamics to downstream effects. As an example, the pressure drop associated with a partially fouled heat exchanger is often not linked to the increased power consumption of the downstream booster compressor. Further, in conventional systems, critical equipment is given much attention in terms of maintaining reliability and availability at an assets level, while performance losses and the impact those losses have on the overall process have generally been overlooked.
Due to the increase in LNG demand seen in recent years, greater emphasis is now being placed on efficiency and performance of liquefaction plants in order to reduce the cost of the delivered gas. Methods and systems that offer such optimized or enhanced operation would be commercially valuable, particularly as they address the several shortcomings found in conventional systems in use today.